1. Field of the Invention
The present invention relates to an air bag system for a vehicle in which there is provided a vent hole in one of base fabric pieces that is a part of an air bag, so that gas flows from the vent hole toward the exterior of the air bag during the expansion of the air bag.
2. Description of the Related Art
There has been known an air bag that inflates and expands by gas supply from an inflator at the time of a collision of a vehicle, thereby to protect an occupant. Such an air bag is provided with a vent hole for discharging gas at an excessive pressure to the exterior of the air bag so as to prevent an excessive rise of gas pressure in the air bag.
If such a vent hole is unblocked at the very initial state of expansion of the air bag, gas supplied from an inflator is leaked from the vent hole, which causes delay of the initial rise of the inner pressure of the air bag. Hence, the vent hole is designed to open on the way of expanding the air bag.
Regarding the air bag with such a vent hole, JP2005-199987A, for example, discloses that there is provided a vent hole cover to block up the vent hole, and one end of the vent hole cover is fixed in the vicinity of the center portion of the second base fabric piece, and the other end thereof is stitched in the vicinity of the outer circumference of the air bag, together with the stitched portion of the first and second base fabric pieces, so as to control the timing when the vent hole comes unblocked.
However, when the vent hole cover stitched in the vicinity of the outer circumference of the air bag comes released into the free state, the vent hole cover instantaneously comes apart from the vent hole, so that gas leakage from a gap between the vent hole cover and the vent hole may occur.
Specifically, before the vent hole cover comes apart from the stitched portion of the vent hole cover so as to unblock the vent hole, that is, before the inner pressure of the air bag is released during the expansion of the air bag, the vent hole cover comes apart from the vent hole just for a moment so that gas in the air bag may leak to the exterior.
To address the above problem, the present invention has an object to provide an air bag system for a vehicle that can maintain the blocking state of a vent hole after a blocking member for blocking up the vent hole comes in the free state and before the inner pressure of an air bag is released, so as to prevent instantaneous gas leakage.